Method of utilizing exit gas from treating ores for engine power



Oct. 25, 1955 P. J. MCGAULE'Y TREATING ORES FOR ENGINE POWER Filed Aug. 12, 1954 AL METHOD OF UTILIZING EXIT GAS FROM INVENTORS Patrick J. MQGuuley Morris Zelenetz BY ATTORNEY United States Patent METHOD OF UTILIZING EXIT GAS FROM TREAT- ING ORES FOR ENGINE POWER Patrick J. McGauley, Port Washington, and Morris Zelenetz, Brooklyn, N. Y., assignors to Chemical Construction Corporation, New York, N. Y., a corporation of Delaware Application August 12, 1954, Serial No. 449,414 Claims. (Cl. 75-97) This invention relates to a process for the production of power. More particularly, it relates to a power producing system. Still more specifically, the invention is concerned with the production of power from those tail gases containing more than 35% water vapor by volume which emanate from a high pressure oxidation leach system wherein an aqueous slurry of an ore or concentrate, capable of producing heat, is subjected to oxidation under superatmospheric pressure.

Prior to the present invention, it has been postulated that sufiicient power could not be produced for commercial use from tail gases which emanate from an oxidation leach autoclave. These gases usually contain essentially water vapor, oxygen, nitrogen and entrained traces of acid droplets and solid particles in varying proportions. At present, it is common practice to vent these tail gases to the atmosphere. This practice results in the loss of readily available power which is wasteful and costly. Although unsuccessful attempts have been made to recover steam from tail gases, no attempt has been made as yet to produce power from the tail gases of an oxidation leach autoclave. It was believed that such tail I gases could not be utilized for the reason that sufiicient power could not be produced therefrom and because they rapidly erode and corrode an expansion engine. Due to to the erosion and corrosion problem, the efiective life of an expansion engine is far too short for commercial practice.

It has been long known, however, that power could be recovered from ofi-gases of nitric acid production by employing ofl-gases to drive an expansion engine. This practice is generally considered inefiicient and impractical, since in nitric acid production the ofi-gases do not contain any substantial amounts of water vapor. Accordingly, the amount of power recovered in nitric acid production has been found to be only about 50% or less of that required for air compression. This small amount of power recovery in nitric acid production has not been considered commercially attractive.

It is a principal object of the invention to produce useful available energy from the tail gases which are exited from an oxidation leach autoclave in a simple and economical manner.

It is a further object of the invention to utilize the tail gases to efliciently drive an expansion engine, without imparting corrosion and erosion thereto, thus utilizing tail gases previously vented to the atmosphere.

In general, the above objects of the invention are accomplished in a simple manner. In the oxidation leach of an ore or concentrate aqueous slurry, tail gases are discharged'from an oxidation leach autoclave and are directed to a tail gas scrubber. The vented exit gases which contain water vapor, oxygen, nitrogen, entrained acidic droplets and solid particles are under superatmospheric pressure and elevated temperatures. The gases are contacted with an aqueous alkaline solution while maintaining the solution at a temperature of substantially that of the tail gases that are introduced into the scrubber.

Any enrained acidic droplets or solid particles in the tail gases that contact the alkaline solution are readily removed therefrom. Where substantial amounts of solid particles are built up in the wash water, the wash water may be bled 01f the system, filtered and the thus filtered water reused. The scrubbed gases are then withdrawn and are used by expanding them in either a reciprocating or rotary expansion engine. This engine drives a generator. Part of the power generated is used to drive a motor-driven compressor which supplies the air required for oxidation. The excess electricity produced can be sold or used where needed. If desired, the expansion engine can drive the compressor as well as a generator directly. Hence, the tail gasescan be readily utilized to produce power.

It has been found that more power surprisingly can be produced from tail gases emanating from an oxidation leach autoclave that is required to compress an oxygenbearing gas, such as oxygen or air, necessary to leach an aqueous slurry of an ore or concentrate which is capable of producing heat upon oxidation, provided the tail gases contain more than 35 water vapor by volume. It has been further found that during the leach operation, water vapor is produced principally due to the heat of reaction. The greater the heat producing components present in the ore or concentrate, the more heat will be produced. This results in a larger volume of water vapor and, consequently, more power. Accordingly, advantage is taken of the heat of reaction during the leaching operation,

It has been further discovered that in order to maintain the water vapor of tail gases of above about 35 by volume, a high operating temperature must be maintained in the oxidation leach autoclave in order to fully utilize the heat of reaction for power recovery through the evaporation of water. Temperatures in the range of from about 300-500 F. or even higher, and advantageously from about 400450 F., are employed. At lower operating temperatures, the tail gases do not have sufiicient water carrying capacity to occlude all the water necessary to effect a heat balance within the autoclave. Some of the heat of reaction in the autoclave must be dissipated with cooling water to obtain the required heat balance at lower operating temperatures. Hence, there is a corresponding loss in power available, since there is a decrease in the volume of water vapor remaining in the tail gases. A good operating pressure range in the autoclave is from 500-1100 pounds per square inch gauge (p. s. i. g.). The temperature and pressure employed in the autoclave will depend on the material being leached and the amount of water employed to make up the ore or concentrate slurry. In practice, it is desirable to employ the highest operating temperature and lowest op erating pressure consistent with good leaching for the specific ore or concentrate being treated. An example of good operating conditions includes a temperature of about 435 F., a pressure of about 600 p. s. i. g. and about a 40% aqueous slurry of the ore or concentrate.

The invention will be further described with reference to the accompanying drawing, the single figure of which is a flow sheet wherein tail gases are treated in accordance with the features of the invention.

Referring to the drawing, an aqueous ore slurry is introduced into a leach feed tank 1. The ore may be an arsenide-ore, but any ore, such as a sulfide ore or concentrate thereof as for example pyrite (FeSz) or chalcopyrite (CuFeSz) may be used, provided the ore is capable of exothermic oxidation. The ore slurry is then intimately mixed in said leach feed tank 1 and Withdrawn by means of a suitable feed pump 2 through a conduit 2a to an oxidation leach autoclave 3 wherein air under superatmospheric pressure is introduced through conduit 11, while agitating the contents therein. The flow of slurry is controlled by an on-off valve 2b. The heat of reaction which follows the introduction of the air under pressure vaporizes the water in the ore slurry because the oxidation leaching process is quite exothermic. In practice, the tail gases are vented to preserve the oxygen partial pressure that is developed in the autoclave during oxidation; The tail gases which are so vented, herein termed exit gases, contain more than 35% by volume of water vapor and a mixture of oxygen and nitrogen, along with entrained traces of acidic droplets and some solid particles. The vented exit gases are then injected into a heat insulated scrubber 4. The scrubber may be any closed vessel of the conventional type. As illustrative types, a plate tower, spray tower tank, packed tower tank, and the like may be mentioned. A heated dilute aqueous alkaline solution, such as for example, sodium carbonate, potassium carbonate, lithium carbonate, sodium hydroxide, potassium hydroxide and equivalents thereof, is sprayed into a scrubber water pump tank 5, throu h any suitable nozzle 14. If the aqueous alkaline solution does not attain the temperature of the tail gases, the temperature may be so increased by incorporating heating coils (not shown) within the scrubber water tank 5. The alkaline solution is then withdrawn and pumped by means of pump 6 to the scrubber 4 through a suitable conduit 6a. The alkaline solution countercurrently contacts the tail gases and neutralizes any acidity Within the tail gases. The scrub solution is continuously recycled through conduits 16 and 12 to scrubber water tank through control valve 12a. Provision may he made to insulate the conduits so as to minimize any heat loss. As the process proceeds in a continuous manner, the scrub water may contain small amounts of solid particles which are entrained in the tail gases. In order to avoid plug ging the spray nozzle in the scrubber, the wash water is filtered or settled by conventional means. In the drawing, a pressure tank 15 is illustrated. The wash water is withdrawn through conduit 16, which extends below a filter 15a in tank 15. It is filtered and reintroduced into the system through conduit 12. The valve 12a controls recycle wash solution flow. Further, additional alkali may be introduced into the scrub water tank 5 through a conduit 17 to insure an alkalinity of a pH of about 8 or even higher. The valve 17a controls the flow of make-up alkali. As a good practice, a constant feed of alkali is introduced into the wash or scrubbing water at 5 in order to maintain the alkalinity in the alkaline range due to the consumption of alkali to neutralize entrained acidic droplets in the tail gases. Any conventional method for ascertaining the pH of the wash water may Y be employed, as by periodically bleeding the wash water system through a conduit 19 and testing for alkalinity. The flow is controlled by a pressure valve 18. Further, any solid material in tank 15 may be bled off or removed through conduit 19.

The temperature within the scrubber 4 is sufficiently high to maintain the water vapor within the tail gases in a vapor state. The pressure within the scrubber 4 is maintained and the temperature therein approximates the temperature of the incoming tail gases. The tail gases are then withdrawn through conduit 13.

The exit temperature of the thus scrubbed tail gases is usually between 400500 F. These gases may be directly led to the expansion engine 8. However, it is preferred to withdraw the tail gases to an indirect heater 7 so that the temperature of these gases may be increased to from about 750-900 F. in order to insure a moisture content of less than 15% in the gas after expansion in a turbine. The indirect heater 7 may be of the conventional type, and advantageously is an oil-fired finned tube heater. The gases are then passed through conduit 13a into an expansion engine 8 which is preferably a steam turbine, although a gas turbine may be used. The turbine is generally made of stainless steel. The tail or exit gases after expansion are vented to the atmosphere through conduit 8a. The turbine drives both a corn: pressor 9 and a generator 21 which are connected to the turbine 8 through a shaft 10. Air is introduced into the air compressor 9, which upon compression develops sufiicient air pressure to effect an oxidation leach. The compressed air is then directed through a suitable conduit 11 to the leach autoclave 3. The generator 21 produces electricity from the power in excess of that required for air compression. This electricity can be distributed by conventional methods as desired.

The oxidized slurry ore in autoclave 3 is exited either continuously or intermittently through conduit 20. The flow of the ore through conduit 20 is controlled by means of a suitable control valve 3a.

The invention will be further illustrated by the following examples, but it is not to be construed as being limited thereto. Unless otherwise specified, the parts in the examples are by weight.

Example I 1000 tons of a gold-arsenide-sulfide ore, analyzing as: l2%-As, 12.5%-S, 6.0%-Fe, 0.001-Au, and 69.5%-insolubles, is slurried with water to prepare a 37% aqueous slurry. The slurry is continuously introduced into an autoclave at the rate of 600 tons per day and vigorously stirred. Prior to the introduction of slurry, however, the autoclave is heated with steam to about 200 F. and the air flow is started. It is introduced at the rate of 890 tons per day wherein the temperature and pressure rise quite rapidly within 30 minutes or less. The slurry is subjected to an air oxidation leach in the autoclave. The pressure of the air introduced into the autoclave is 600 p. s. i. g. An equilibrium temperature of 435 F. is developed in the autoclave within 30 minutes or less due to the exothermic nature of the reaction. In order to maintain this temperature and pressure, tail gases resulting from the heat of the reaction in the oxidation leach autoclave are continuously exited or vented off. However, the tail or exit gases contain entrained traces of acidic droplets and some solid particles along with 56% by volume of water vapor and 44% by volume of a mixture of oxygen and nitrogen. These gases are injected into the lower portion of a scrubber and contacted countercurrently with an aqueous sodium carbonate solution of about pH 10 which is sprayed into the scrubber. The alkaline solution, however, was preferably heated to a temperature of about 435 F. prior to introduction into the scrubber. The aqueous alkaline solution after countercurrently contacting the tail gases is withdrawn, recycled and reused. After one hour of operation, the pH dropped to below about 7.5, and additional alkali is added to the wash solution to insure a pH greater than about 8. In this connection, a good operating pH range is from about 8 to 12. After five hours of operation, the wash water is filtered so as to remove occluded solids and the latter water reused.

The gases are countercurrently scrubbed and exited. They are introduced into an oil-fired finned tube heater so as to increase their temperature to 750 F. These gases are withdrawn from that heater and are expanded through a turbine of stainless steel. The turbine drives a generator which develops 7550 horsepower. An air compressor converts atmospheric air to superatmospheric air of 650 p. s. i. g., which allowsfor a 50 p. s. i. g. drop in the system. The compression of air for oxidation in the leach autoclave requires 5500 horsepower. Accordingly, an excess of 2050 horsepower is produced. The compressed air is then introduced into the leach autoclave.

Example 2 In this run, the procedure of Example 1 is repeated except that 75% excess air required to effect the oxidation of the ore slurry is employed under a total pressure of 600 p. s. i. g. An equilibrium temperature of 410 F.

is obtained. The composition of the exit gases is found to be 42.5% water vapor by volume, 57.5% mixture of oxygen and nitrogen by volume along with entrained traces of acidic droplets and solid particles. The tail gases are scrubbed as in Example 1 and preheated to 750 F. They drive a turbo-generator which develops 9100 horsepower (H. P.). The compression of air required for the oxidation of the ore is 8250 H. P. Hence, an excess of 850 horsepower is produced.

Example 3 The procedure of Example 1 is repeated. However, in this example, 135% excess of air required to eifect the oxidation of the ore slurry is introduced in the autoclave maintained at a pressure of 600 p. s. i. g. An equilibrium temperature of 392 F. is obtained. Under these conditions of temperature and pressure, the composition of the exit gases is found to be 34.4% water vapor by volume and 65.6% mixture of oxygen and nitrogen by volume along with entrained traces of acidic droplets and solid particles. After scrubbingthe gases as outlined in Example 1, they are preheated to 750 F. and drive a turbo-generator which develops 10,900 horsepower. To operate the leach oxidation autoclave at a temperature of 392 F., compression of air for oxidation requires 11,000 horsepower. Accordingly, there is insufficient power produced due to the fact that the exit gases contained 34.5% water vapor by volume.

Example 4 In this example, a temperature of 375 F. and pressure of 600 p. s. i. g. are maintained for the oxidation conditions in the autoclave of Example 1 which is repeated. The temperature in the autoclave is obtained by external water cooling. The exit gas composition is found to be essentially 71.2% of a mixture of oxygen and nitrogen by volume and 28.8% water vapor by volume. After scrubbing these gases to remove entrained acidic droplets and solid particles and preheated to 750 F., they drive a turbo-generator which develops 4550 H. P. However, the power required for air compression to eifect the oxidation leach in the autoclave is 5500 H. P., which is 950 H. P. shy of the power required for air compression.

These examples demonstrate the criticality of the water vapor which must be present in the exit gases which is in excess of about 35% by volume. Further, these examples illustrate that in accordance with the process of the invention, more power than is required to compress an oxygen-bearing gas is surprisingly produced.

We claim:

1. In producing utilizable power from exit gas mixtures consisting essentially of water vapor in excess of about 35% by volume and a mixture of oxygen and nitrogen, containing entrained traces of acidic droplets and solid particles, discharged from a high pressure oxidation leach system wherein an aqueous slurry is prepared from an ore capable of exothermic oxidation and said slurry is subjected to an oxygen-bearing gas under superatmospheric pressure of at least 500 pounds per square inch gauge and at a temperature of at least 300 F. whereby a finite quantity of oxygen-bearing gas compressed to at least said superatmospheric pressure is consumed; the steps which comprise: introducing said exit gas mixtures into a vessel at a temperature in excess of about 300 F. and under a superatmospheric pressure, in excess of 500 pounds per square inch, removing entrained traces of acidic droplets and solid particles from said gases, expanding said gases in an expansion engine to generate power in excess of that required to compress said finite quantity of oxygen-bearing gas to said pressure, and compressing said oxygen-bearing gas.

2. A method in accord with claim 1 wherein the oxygen-bearing gas is air.

3. A method in accord with claim 1 wherein the water vapor is about 56% by volume of the exit gas mixtures.

4. A method in accord with claim 1 wherein the gas mixtures are preheated to a temperature between about 750-900 F. prior to their introduction into the expansion engine.

5. A method in accord with claim 1 wherein the ore is a gold sulfide ore.

6. In producing utilizable power from exit gas mixtures consisting essentially of water vapor in excess of about 35 by volume and a mixture of oxygen and nitrogen containing entrained traces of acidic droplets and solid particles discharged from a high pressure oxidation leach system wherein an aqueous slurry is prepared from an ore capable of exothermic oxidation and said slurry is subjected to an oxygen-bearing gas under superatmospheric pressure of at least 300 F. whereby a finite quantity of oxygen-bearing gas compressed to at least said superatmospheric pressure is consumed; the steps which comprise: introducing said exit gas mixtures into a vessel at a temperature in excess of about 300 F. and at a superatmospheric pressure in excess of 500 pounds per square inch gauge, contacting said exit gases countercurrently with a heated aqueous alkaline solution, maintaining the alkalinity of the latter solution at a pH greater than about 7.5, withdrawing resultant gases substantially free from acidic droplets and solid particles from said vessel, expanding a suflicient amount of withdrawn gases in an expansion engine to generate power in excess of that required to compress said finite quantity of oxygen-bearing gas to said pressure, and compressing said oxygen-bearing gas.

7. A method in accord with claim gen-bearing gas is air.

8. A method in accord with claim 6 wherein the tail gases are introduced into the scrubber at a pressure of about 600 pounds per square inch gauge and at a temperature of about 435 F.

9. A method in accord with claim 6 wherein the water vapor is about 56 by volume of the exit gas-mixtures.

10. A method in accord with claim 6 wherein the gas mixtures are preheated to a temperature between about 750-900 F. prior to their introduction into the expansion engine.

11. A method in accord with claim 6 wherein the ore is a gold arsenide sulfide ore.

12. A method of treating an ore capable of exothermic oxidation and the production of utilizable power therefrom which comprises the steps of: forming an aqueous slurry of said ore, introducing said slurry into a closed vessel, subjecting said slurry to an oxygen-bearing gas under superatmospheric pressure of at least 500 pounds per square inch gauge and at a temperature of at least 300 F. whereby a finite quantity of oxygen-bearing gas compressed to at least said superatmospheric pressure is consumed, withdrawing from said vessel exit gas mixtures consisting essentially of water vapor in excess of about 35 by volume and a mixture of oxygen, nitrogen, and entrained traces of acidic droplets and solid particles, removing said entrained matter from the gas mixtures, expanding the latter in an expansion engine to generate power in excess of that required to compress said finite quantity of oxygen-bearing gas, withdrawing resultant treated ore slurry from the closed vessel, and introducing said compressed gas to said closed vessel to further eifect exothermic oxidation of aqueous ore slurry.

13. A method in accord with claim 12 wherein the ore is a gold arsenide sulfide ore.

14. A method in accord with claim 12 wherein the oxygen-bearing gas is air.

15. A method in accord with claim 12 wherein the entrained matter is removed from the exit gas mixtures by C(fiiltficfing said exit gases countercurrently with aqueous a ali.

6 wherein the oxy- No references cited. 

1. IN PRODUCING UTILIZABLE POWER FROM EXIT GAS MIXTURES CONSISTING ESSENTIALLY OF WATER VAPOR IN EXCESS OF ABOUT 35% BY VOLUME AND A MIXTURE OF OXYGEN AND NITROGEN, CONTAINING ENTRAINED TRACES OF ACIDIC DROPLETS AND SOLID PARTICLES, DISCHARGED FROM A HIGH PRESSURE OXIDATION LEACH SYSTEM WHEREIN AN AQUEOUS SLURRY IS PREPARED FROM AN ORE CAPABLE OF EXOTHERMIC OXIDATION AND SAID SLURRY IS SUBJECTED TO AN OXYGEN-BEARING GAS UNDER SUPERATMOSPHERIC PRESSURE OF AT LEAT 500 POUNDS PER SQUARE INCH GAUGE AND AT A TEMPERATURE OF AT LEAST 300* F. WHEREBY A FINITE QUANTITY OF OXYGEN-BEARING GAS COMPRESSED TO AT LEAST SAID SUPERATMOSPHERIC PRESSURE IS CONSUMED; THE STEPS WHICH COMPRISE: INTRODUCING SAID EXIT GAS MIXTURES INTO A VESSEL AT A TEMPERATURE IN EXCESS OF ABOUT 300* F. AND UNDER A SUPERATMOSPHERIC PRESSURE IN EXCESS OF 500 POUNDS PER SQUARE INCH, REMOVING ENTRAINED TRACES OF ACIDIC DROPLETS AND SOLID PARTICLES FROM SAID GASES, EXPANDING SAID GASES IN AN EXPANSION ENGINE TO GENERATE POWER IN EXCESS OF THAT REQUIRED TO COMPRESS SAID FINITE QUANTITY OF OXYGEN-BEARIING GAS TO SAID PRESSURE, AND COMPRESSING SAID OXYGEN-BEARING GAS. 